应该选A.他刚刚被任命为领导.
如果选B 任命别人是领导的话,那么appoint sb as as a leader,所以这句话改为
He has just appointed Mr Li as the leader of the Model Group...
appoint及物动词 vt.
1.任命, 委派
He was appointed secretary.
他被任命为秘书.
2.确定, 指定; 约定
We must appoint a day to meet again.
我们要约定好下次会面的日期.
He appointed the schoolhouse as the place for the meeting.
他指定校舍为开会地点.
The head of the workshop appointed that the work should be finished in two days.
车间主任下令这项工作必须在两天之内完成.